The seat was created in 1983, as part of an expansion of Northern Ireland's constituencies from 12 to 17, and was predominantly made up from parts of South Antrim and North Down. In their original proposals, in January 1980, the boundary commission proposed calling it 'Lagan'. In further revisions in 1995 it lost some areas to both Belfast West and Strangford. Currently the constituency contains most of Lisburn district and part of Banbridge district.
Following their review of all parliamentary seats in Northern Ireland prior to the United Kingdom general election, 2010 the Boundary Commission for Northern Ireland made alterations to Lagan Valley. In an unprecedented decision, passed by Parliament through the Northern Ireland Parliamentary Constituencies Order, one electoral ward was split between two constituencies. This followed concerns in Derryaghy about being moved into the neighbouring West Belfast seat.
The areas making up Lagan Valley since 2010 are;
From Lisburn City government area; Ballinderry, Ballymacash, Ballymacbrennan, Ballymacoss, Blaris, Dromara, Drumbo, Harmony Hill, Hilden, Hillhall, Hillsborough, Knockmore, Lagan Valley, Lambeg, Lisnagarvey, Maghaberry, Magheralave, Maze, Moira, Old Warren, Seymour Hill, Tonagh, and Wallace Park. The area of Derryaghy to the south and east of its boundary with Lagmore is also within this seat.
Dromore North, Dromore South, Gransha, and Quilly, from the Banbridge district